In recent years, the miniaturization of electronic equipment typified by portable terminals and the like has advanced increasingly, and therefore connectors used in it have a significant tendency to a narrower pitch, lower height, and narrower width. A smaller connector has narrower pin width and a small folded work shape, and therefore high strength for obtaining necessary spring properties is required of the member used. In this respect, a copper alloy containing titanium (hereinafter referred to as “copper titanium alloy”) has relatively high strength and has the most excellent stress relaxation properties among copper alloys and therefore has been used from old times as a member for a signal system terminal of which strength is particularly required.
The copper titanium alloy is an age-hardenable copper alloy. When a supersaturated solid solution of Ti that is a solute atom is formed by solution treatment, and heat treatment is performed from the state at low temperature for a relatively long time, a modulation structure that is periodical fluctuations of Ti concentration develops in the matrix phase by spinodal decomposition, and the strength improves. At this time, the problem is that strength and bending workability are conflicting properties. In other words, when the strength is improved, the bending workability is impaired, and on the contrary, when the bending workability is regarded as important, the desired strength is not obtained. Generally, as the draft of cold rolling is increased, introduced dislocations increase, and the dislocation density increases, and therefore nucleation sites contributing to precipitation increase, and the strength after aging treatment can be increased. But, when the draft is increased too much, the bending workability worsens. Therefore, achieving both strength and bending workability has been considered as a problem.
Therefore, techniques are proposed in which attempts are made to achieve both the strength and bending workability of the copper titanium alloy from the perspectives of adding third elements such as Fe, Co, Ni, and Si (Patent Literature 1), restricting the concentration of a group of impurity elements dissolved in a matrix phase and precipitating these as second-phase particles (Cu—Ti—X-based particles) in a predetermined distribution form to increase the regularity of a modulation structure (Patent Literature 2), prescribing slight amounts of added elements effective in making crystal grains finer and the density of second-phase particles (Patent Literature 3), making crystal grains finer (Patent Literature 4), controlling crystal orientation (Patent Literature 5), and the like.
In addition, in Patent Literature 6, it is described that as a titanium modulation structure due to spinodal decomposition develops, the fluctuations of titanium concentration increase, and thus tenacity is given to a copper titanium alloy, and the strength and the bending workability improve. Therefore, in Patent Literature 6, a technique of controlling the fluctuations of Ti concentration in a matrix phase due to spinodal decomposition is proposed. In Patent Literature 6, it is described that after final solution treatment, heat treatment (under aging treatment) is introduced to previously induce spinodal decomposition, and then cold rolling at a conventional level and aging treatment at a conventional level or aging treatment with a lower temperature and a shorter time than those of the aging treatment at a conventional level are performed to increase the fluctuations of Ti concentration and achieve higher strength of a copper titanium alloy.